A wave of political turmoil crashed over Spain on Thursday as Prime Minister Pedro Sánchez publicly weighed resigning his post after a judge agreed to investigate his wife over allegations that he and other officials decried as a politically driven smear campaign.

The judge’s decision to take up the case — which was brought by a self-described anti-graft group on the basis of online news reports about alleged influence peddling — prompted Mr. Sánchez’s supporters to coalesce behind him and public prosecutors to move quickly on Thursday to try to get the case dismissed.

Mr. Sánchez, whose political survival skills have for years astonished his supporters and detractors alike, wrote in a public letter Wednesday that the accusations against his wife, Begoña Gómez, were false and amounted to harassment. One of the most prominent leftist leaders in Europe, Mr. Sánchez has canceled his public schedule while he reflects on his next move. He plans to address the nation on Monday.

As Mr. Sánchez holed up with his family and resisted the entreaties of his allies to hit the campaign trail before key elections in the Catalonia region and for the European Parliament, supporters talked about mobilizing rallies to persuade him to stay.

And a wide array of Spaniards, from the political elite to citizens on the streets, expressed bewilderment at the uncharacteristic retreat by a prime minister who only recently had reclaimed his post in elections last summer, and by the country’s odd state of affairs.

“It’s a mess,” said Pablo Simón, a political scientist at Carlos III University in Madrid, who said he was struck by the deeply personal tone of Mr. Sánchez’s letter. He added that the investigation into Mr. Sánchez’s wife of 18 years had apparently set off an emotional reaction because, politically speaking, “there were no clear incentives for this gambit; it’s very risky.”

“He has done something with no precedent in a democracy,” Mr. Simón added, suggesting that the prime minister was betting the public would find the investigation against his wife so outrageous as to prompt a national reckoning. Mr. Sánchez sought a “social confidence vote,” he said, in which he is calling on the public, the media and even the establishment opposition to take sides and decide, “Do you consider this acceptable?”

The trigger for the sudden crisis was the decision by a Spanish judge to entertain a complaint from Clean Hands, a group known for filing cases in court against politicians and other prominent Spaniards.

The group filed a complaint accusing Ms. Gómez of influence peddling and corruption — citing as potential evidence online news reports that it has acknowledged could contain false information. The judge ordered a preliminary investigation based on those online media reports.

Two of the articles allege that in 2020, Ms. Gómez signed two letters of recommendation to support a bid for a public contract by a group of companies to which she has personal and professional ties. The articles claim that the main stakeholder of the group designed the master’s program that Ms. Gómez ran at Complutense University of Madrid and that the companies supported by Ms. Gómez competed with 20 rivals and won three contracts worth more than 10 million euros, or about $10.7 million.

The complaint by Clean Hands also cited an article in the online media outlet El Confidencial that claimed Ms. Gómez met with representatives of Air Europa, a Spanish airline, in 2020 to sign a confidential agreement in which the airline would pay 40,000 euros a year ($43,000) to the Africa Center she led at a private university. Months later, the airline received more than €400 million in bailout funds during the pandemic.

In a statement, the Africa Center denied it had “ever received financial contributions” from Air Europa’s parent company or affiliates. It said the Center signed during Ms. Gómez’s tenure in 2020 a sponsorship deal with the airline’s parent company that included four airline tickets to a work event in London, which was “never executed” because of the pandemic. It said Ms. Gómez’s 2018 contract specifically prevented the Center from benefiting from her “family position.”

The Spanish press has widely reported that one of the news reports cited by Clean Hands has already been shown to be wrong. The online paper The Objective accused the government of burying the information that a subsidy had been awarded to the prime minister’s wife — but it turns out that the recipient of the subsidy was a businesswoman in the catering industry who shares the same name as Ms. Gómez.

The judge has summoned two journalists to testify about their reporting. Ms. Gómez, the wife of Mr. Sánchez, has not been summoned and has not commented on the complaint.

The government, however, has called the Clean Hands complaint groundless, arguing that Ms. Gómez did nothing irregular or improper, and that headlines linking the prime minister to corruption fed into the hands of the opposition that had slung the mud in the first place.

On Thursday, Spain’s independent prosecuting authority made what it called a “direct appeal” to the Provincial Court in Madrid to dismiss the preliminary investigation.

Miguel Bernad, the leader of Clean Hands, has acknowledged that the complaint could be based on false information.

“It will now be the judge who must verify whether said journalistic information is true or not,” he wrote in a statement.

Mr. Sánchez wrote in his public letter that the accusations against his wife, who played a key role in his political ascent, were not true.

“We have been denying the falsehoods expressed, while Begoña has taken legal action so that these same digital companies rectify what, we maintain, is spurious information,” he wrote, adding it was “an operation of harassment and demolition by land, sea and air to try to weaken me politically and personally by attacking my wife.”

In Spain, individual citizens in groups like Clean Hands can bring legal complaints even when they are not personally involved and have suffered no damages. The group’s website describes its main purpose as filing “all types of complaints against political or economic corruption that harms the public or general interest.”

Spain’s National Court in 2021 found the group guilty of using smear campaigns to extort banks and companies. Spain’s Supreme Court overturned the decision because it said no crime was committed, but called the group’s methods “reprehensible.”

Deputy Prime Minister Yolanda Díaz, the leader of the left-wing Sumar coalition, echoed the complaints of a smear campaign on Wednesday and said that right-wing forces could “not be allowed to win.”

But Spain’s main conservative opposition party, still smarting from its failure to form a government despite having received more votes than Mr. Sánchez, who outflanked them by building a broader coalition, seized the chance to pile on a political enemy.

Accusing Mr. Sánchez of engaging in victimization for political gain, the center-right Popular Party insisted the prime minister tell all about “the scandals surrounding his party, his government and his partner.”

If Mr. Sánchez resigns, several procedures are likely, experts said.

His government would assume a caretaker status until Parliament agreed on a new candidate to try to cobble together a governing coalition. Mr. Sánchez could also ask Parliament to decide whether he should remain through a confidence vote requiring only a simple majority.

Mr. Sánchez could also call another snap election, as he did after his party took a shellacking in regional elections last year.

At that time, he managed to gather enough support to block the formation of a government by the center-right Popular Party and the far-right Vox party. He then pieced together a parliamentary majority out of the fractious, and often opposing, other parties.

But calling a snap election would carry risks, especially since the latest polls show his Socialist Party trailing the Popular Party.